Radiochromic film dosimetry with flatbed scanners: A fast and accurate method for dose calibration and uniformity correction with single film exposure

Abstract

Film dosimetry is an attractive tool for dose distribution verification in intensity modulated radiotherapy (IMRT). A critical aspect of radiochromic film dosimetry is the scanner used for the readout of the film: the output needs to be calibrated in dose response and corrected for pixel value and spatial dependent nonuniformity caused by light scattering; these procedures can take a long time. A method for a fast and accurate calibration and uniformity correction for radiochromic film dosimetry is presented: a single film exposure is used to do both calibration and correction. Gafchromic EBT films were read with two flatbed charge coupled device scanners (Epson V750 and 1680Pro). The accuracy of the method is investigated with specific dose patterns and an IMRT beam. The comparisons with a two-dimensional array of ionization chambers using a 18 x 18 cm2 open field and an inverse pyramid dose pattern show an increment in the percentage of points which pass the gamma analysis (tolerance parameters of 3% and 3 mm), passing from 55% and 64% for the 1680Pro and V750 scanners, respectively, to 94% for both scanners for the 18 x 18 open field, and from 76% and 75% to 91% for the inverse pyramid pattern. Application to an IMRT beam also shows better gamma index results, passing from 88% and 86% for the two scanners, respectively, to 94% for both. The number of points and dose range considered for correction and calibration appears to be appropriate for use in IMRT verification. The method showed to be fast and to correct properly the nonuniformity and has been adopted for routine clinical IMRT dose verification.

Dosimetric characterization and use of GAFCHROMIC EBT3 film for IMRT dose verification

“…To minimize the effect of the lateral dependence artifacts (the nonuniform response of the readout due to the light scattering of the scanner lamp caused by particles in the film active layer ( 20 ) , a $10\times 12.5\hspace{0.17em}{\text{cm}}^2$ cardboard template was fitted to the scanner to position films at a reproducible central location of the scan surface that can be considered uniform. ( 16 ) To confirm this assumption, five OD measurements over the scanner central area were performed, resulting in a standard error less than 0.06%.…”

“…( 20 ) RGB‐positive images were collected at a depth of 16 bits per color channel with a spatial resolution of 72 dpi corresponding to a pixel size of $0.35\times 0.35\hspace{0.17em}{\text{mm}}^2$, and saved in tiff format.…”

On using the dosimetric leaf gap to model the rounded leaf ends in VMAT/RapidArc plans

“…The investigated films were placed on the scanner glass in the same orientation as the calibration films because the sensitivity of GAFCHROMIC film is known to depend on the film orientation on the scanner glass (22) . To compensate for the dependence of the readings on the location at the scanner glass and similarly to the protocol developed by Menegotti, (23) a position‐dependent multiplicative correction function (an inverted‐Gaussian along the light source bar direction of the scanner and no correction across the light source bar) was applied to the pixel intensities. In addition, the pieces of film for measuring the dose profiles were placed such that the investigated profile was lined up along the direction of movement of the light source (where the readings do not depend on the position on the scanner glass).…”

Initial testing of a pixelated silicon detector prototype in proton therapy

“…Chief of these is that postexposure processing limits radiochromic film's ability to provide real‐time data 7. In addition, artifacts can be introduced in dose measurements due to properties of the film itself (e.g., variations in active layer or substrate thickness, postexposure intensification), environmental and handling effects (e.g., temperature, light sensitivity), and scanner response (e.g., lateral position artifact,8 film orientation,9 dust, fingerprints, film curl, and local‐, inter‐ and intrascanner factors) 7. Finally, inter‐ and intrafilm lot and scanner variation can result in inconsistent dose mapping between film lots, scanner models, and environmental processing conditions 10.…”